In U.S. Pat. No. 4,113,360 issued Sep. 12, 1978 to Bauer et al., titled “INDICATING DEVICE FOR ILLUSTRATING SYMBOLS OF ALL KINDS,” a display device is disclosed comprising a first plate acting as a light guide or fluorescent material, a second plate positioned some distance apart from the first plate, and a thin movable film situated between the two plates. The movable film is flexible and can be made to locally contact the first plate and allow light to be transmitted from the first plate into the film. If the film is constructed to scatter the light, then the movable film acts as an optical switch to create bright and dark regions on the plates as the film contacts or separates from the first plate, respectively. Rapid contact and separation of the film from the first plate can be used to create gray regions.
Bauer et al. teach controlling the film's movement by electrical means. For example, the film may contain a very thin layer of indium tin oxide that permits an electrical charge to be applied to the film. Similar conductive layers may be placed on the plates. An electrical bias between the plates and the film may be used to move the film toward or away from the light guide. Alternatively, U.S. Pat. No. 5,771,321 issued Jun. 23, 1998 to Stern, titled “MICROMECHANICAL OPTICAL SWITCH AND FLAT PANEL DISPLAY,” describes an electromechanical means of controlling the film's movement.
Typically, the plates are rigid with a thickness on the order of millimeters and are comprised of clear materials such as glass or hardened plastic. The film, on the other hand, must be flexible and it has thickness on the order of a micron. The film may be comprised of resin material such as polycarbonate or polystyrene as suggested by Stem in U.S. Pat. No. 5,771,321, referenced above.
One drawback to the operation of an information display panel using the optical switching device described above, is that the movement of the optical film may be impeded by an air pressure differential in the spaces existing between the film and the plates. To overcome the air pressure differential, undesirably high voltages are required to move the film. In World Intellectual Property Organization Application Publication No. WO 99/28890, by Gerardus Van Gorkom, published on Jun. 10, 1999, and titled “DISPLAY DEVICE COMPRISING A LIGHT GUIDE,” a means of minimizing pressure differential is proposed whereby the film is situated in an evacuated space. Van Gorkom discloses applying a vacuum of preferably less than 10 Torr to the chambers inside the switching device. However, a highly evacuated system is difficult to fabricate and is vulnerable to air leakage during the lifetime of the switching device operating at ambient conditions. Moreover, an evacuated system precludes the use of plastic plates in the switching device since plastic materials are permeable to ambient gases such as nitrogen, oxygen, and water. Because rigid glass plates would be required to maintain a vacuum inside the switching device, a flexible plastic display panel is not possible using Van Gorkom's teachings. Therefore, it remains highly desirable to have an optical switching device that does not require an evacuated system.
Another drawback to the preparation of the optical switch film described above is the need to apply a conductive layer to the optical switch film in a separate operation. Typically, the conductive material is a thin, transparent coating of indium tin oxide that is separately applied at high temperatures of 80–200 degrees Celsius. At these high temperatures, many polymeric films are vulnerable to thermal degradation and/or mechanical distortion. Therefore, it is desirable to have a method of manufacturing an optical switch film with an electrically conductive layer that does not require a separate manufacturing operation and that does not require a high temperature application process.
The flexible optical switch film described above is generally desired to have good light scattering ability, transparency, high uniformity, and low birefringence. Moreover, optical switch films are generally very thin (i.e. on the order of a micron in thickness), but their thickness may vary depending on the final application.
In general, optical films made from polymer resins are prepared either by melt extrusion methods or by casting methods. Melt extrusion methods involve heating the resin until it is molten (i.e., approximate viscosity on the order of 100,000 cp); applying the hot molten resin to a highly polished metal band or drum with an extrusion die; cooling the film; and finally peeling the film from the metal support. For many reasons, however, films prepared by melt extrusion are generally not suitable for some optical applications. Principal among these is the fact that melt extruded films exhibit a high degree of optical birefringence. In the case of many polymers there is the additional problem of melting the polymer. For example, highly saponified polyvinyl alcohol has a very high melting temperature of 230 degrees Celsius, and this is above the temperature where discoloration or decomposition begins (˜200 degrees Celsius). Similarly, cellulose triacetate polymer has a very high melting temperature of 270–300 degrees Celsius, and this is above the temperature where decomposition begins. In addition, melt extruded films are known to suffer from other artifacts such as poor flatness, pinholes, and inclusions. Such imperfections may compromise the optical and mechanical properties of optical films. For these reasons, melt extrusion methods are generally not suitable for fabricating many polymer resin films intended for optical applications. Rather, casting methods are generally used to produce these films.
As stated above, polymer resin films, for optical applications, are manufactured almost exclusively by casting methods. Casting methods involve initially dissolving the polymer in an appropriate solvent to form a dope having a high viscosity on the order of 50,000 cp; applying the viscous dope to a continuous highly polished metal band or drum through an extrusion die; partially drying the wet film; peeling the partially dried film from the metal support; and finally, conveying the partially dried film through an oven to more completely remove solvent from the film. Cast films typically have a final dry thickness in the range of 40–200 μm.
In general, thin films of less than 40 μm are very difficult to produce by casting methods due to the fragility of the wet film during the peeling and drying processes. Films having a thickness of greater than 200 μm are also problematic to manufacture due to difficulties associated with the removal of solvent in the final drying step. Although the dissolution and drying steps of the casting method add complexity and expense, cast films generally have better optical properties when compared to films prepared by melt extrusion methods, and problems associated with high temperature processing are avoided.
One obvious drawback to using casting methods to prepare optical switch films is that very thin films of less than 40 microns are very difficult to manufacture in practical operations. Another drawback to the casting method is the inability to accurately apply multiple layers. As noted in U.S. Pat. No. 5,256,357 issued Oct. 26, 1993 to Hayward, titled “APPARATUS AND METHOD FOR COCASTING FILM LAYERS,” conventional multi-slot casting dies create unacceptably non-uniform films. In particular, line and streak nonuniformity is greater than 5% with prior art devices. Acceptable two layer films may be prepared by employing special die lip designs as taught by Hayward in U.S. Pat. No. 5,256,357, but the die designs are complex and may be impractical for simultaneously applying more than two layers.
The manufacture of polymer resin films by the casting method is also confounded by a number of artifacts associated with the peeling and conveyance operations. Peeling operations, for example, frequently require using converting aids such as special co-solvents or additives in the casting formulation to facilitate peeling the film from the metal substrate without creating streak artifacts. In fact, peeling can be so problematic that some films such as polymethylmethacrylate films can not be manufactured by casting methods without resorting to specialty co-polymers as noted in U.S. Pat. No. 4,584,231 issued Apr. 22, 1986, titled “SOLVENT CAST ACRYLIC FILM,” and U.S. Pat. No. 4,664,859 issued May 12, 1987, titled “PROCESS FOR SOLVENT CASTING A FILM,” both issued to Knoop. In addition to the aforementioned peeling artifacts, cast films may be damaged during conveyance operations while traveling across numerous rollers prior to the final drying operation. For example, abrasion, scratch and wrinkle artifacts of polycarbonate films have been noted in U.S. Pat. No. 6,222,003 issued Apr. 24, 2001 to Hosoi et al., and titled “OPTICAL FILM AND METHOD FOR PRODUCING SAME.” To minimize damage during conveyance, cast polycarbonate films require additional attention be spent on the film, including using special additives that act as lubricants or surface modifiers, or using a protective laminate sheet, or using knurled film edges. However, special additives may compromise film clarity. Moreover, lamination and edge knurling devices are expensive and add complexity to the casting process.
Finally, cast films may exhibit undesirable cockle or wrinkles. Thinner films are especially vulnerable to dimensional artifacts, either during the peeling and drying steps of the casting process, or during subsequent handling of the film. Very thin films on the order of one micron in thickness are especially difficult to handle without wrinkling. Moreover, many cast films may naturally become distorted over time due to the effects of moisture. For optical films, good dimensional stability is necessary during storage as well as during subsequent fabrication of the switching device.
Consequently, a need exists to overcome the limitation of requiring an evacuated system in optical switch devices. A specific need exists to minimize the pressure differential across the film during movement of the film. Yet another need exists to overcome the inherent necessity of a separate high temperature process when applying an electrically conductive layer to an optical switch film.